1. Field of the Invention
The present invention relates to a method for producing a mold for minute or fine pattern transfer used for nanoimprint and the like, a method for producing a diffraction grating using the same, and a method for producing an organic EL element an organic EL element (Organic Electro-Luminescence element or organic light emitting diode) including the diffraction grating as well as the mold for the minute pattern transfer, the diffraction grating, and the organic EL element, those of which are obtained by using the methods described above.
2. Description of the Related Art
There has been known a lithography method as a method for forming a minute pattern such as a semiconductor integrated circuit. The resolution of the pattern formed by the lithography method is dependent on the wavelength of a light source and/or the numerical aperture of an optical system, and a light source having shorter wavelength is expected in order to meet demand for miniaturized devices in recent years. However, the light source having the short wavelength is expensive, development thereof is not easy, and development of an optical material transmitting such a short-wavelength light is also needed. Further, manufacture of a large-area pattern by a conventional lithography method requires a large-size optical element, thereby having difficulties in technical and economic aspects. Thus, a novel method for forming a desired pattern having a large area has been considered.
There has been known a nanoimprint method as the method for forming the minute pattern without using a conventional lithography apparatus. The nanoimprint method is a technique such that a pattern of an order of nanometer can be transferred by sandwiching resin between a mold and a substrate. The nanoimprint method basically includes four steps of: i) application of a resin layer; ii) press of the mold; iii) pattern transfer; and iv) mold-releasing. The nanoimprint method is excellent in that processing on a nanoscale can be realized by the simple process as described above. Further, an apparatus used in the nanoimprint method is simple or easy, is capable of performing a large-area processing, and promises a high throughput. Thus, the nanoimprint method is expected to come into practical use in many fields such as a storage medium, an optical member, and a biochip, in addition to a semiconductor device.
However, even in the nanoimprint method, a mold for transferring a pattern having a line width of tens of nanometers basically requires that the pattern of a resist (resist pattern) on a silicon substrate is exposed and then is developed with using the lithography apparatus. A current seed layer made of metal is formed on the resist pattern to perform electroforming of the mold with using the obtained resist pattern. However, in a case that definition of the pattern is not more than 100 nm, coatability or coverage of the current seed layer formed on the resist pattern by a sputtering is decreased, and the film thickness of the current seed layer obtained varies among the upper portion, the sidewall, and the bottom portion (substrate exposed portion in a concave portion of the pattern, that is a trench) of the resist pattern. Especially, a problem arises such that formation of the current seed layer preferentially proceeds at the upper portion of the resist pattern to cause a narrowing of a trench opening. Thus, in a case that a hole or trench and a ridge are formed on the substrate by using a resist layer, there is a problem such that metal is less likely to be deposited on the bottom portion of the hole or trench during the formation of the current seed layer; and overhang is caused at the upper portion of the ridge of the resist layer. In a case that the electroforming process of a stacked body is performed by using such a current seed layer, an electroformed layer is joined to the upper part of the hole or trench due to the overhang and a void is left inside the trench. As a result, the mold obtained by the electroforming has a problem such that the mold has low mechanical strength and therefore defects such as deformation of the mold and pattern defect are caused.
In order to solve the problem(s) as described above, Japanese Patent Application Laid-open No. 2010-017865 discloses a method for manufacturing a mold for nanoimprint including the steps of: forming a resist layer which includes a concavity and convexity pattern on a substrate having a conductive surface and then exposing the conductive surface at a concave portion of the concavity and convexity pattern of the resist layer; performing electroforming on the conductive surface exposed at the concave portion of the concavity and convexity pattern of the resist layer and then forming an electroformed layer having a film thickness greater than that of the resist layer; and removing the substrate having the conductive surface and the resist layer. According to this method, it is possible to grow the electroformed layer in one direction, that is, in an upward direction from the conductive surface of the bottom portion of the resist pattern, without using the current seed layer, and thus it is considered that no void exists inside the mold for nanoimprint. However, even this method has still been forced to depend on the lithography method to make the mold used for the nanoimprint method.
The inventors of the present invention disclose, in International Application No. PCT/JP2010/62110, a method for obtaining a mold having a minute and irregular concavity and convexity pattern formed therein by applying a block copolymer solution including a block copolymer and a solvent, satisfying a predetermined condition, onto a base member; and performing drying to form a micro phase separation structure of the block copolymer. According to this method, it is possible to obtain the mold used for the nanoimprint and the like by using a self-organizing phenomenon of the block copolymer without using the lithography method. A mixture of a silicone-based polymer and a curing agent is dripped onto the obtained mold and then cured to obtain the transferred pattern. Then, a glass substrate to which a curable resin has been applied is pressed against the transferred pattern and the curable resin is cured by irradiation with ultraviolet rays. In this way, a diffraction grating in which a transferred pattern is duplicated is manufactured. It has been confirmed that an organic EL element obtained by stacking a transparent electrode, an organic layer, and a metal electrode on the diffraction grating has sufficiently high light emission efficiency, sufficiently high level of external extraction efficiency, sufficiently low wavelength-dependence of light emission, sufficiently low directivity of light emission, and sufficiently high power efficiency.
It is desired to take the method for producing the diffraction grating achieved in International Application No. PCT/JP2010/62110 by the inventor(s) of the present invention one step further, so as to improve the method to be suitable for mass production of a product including the organic EL element and the like.
In view of the above, an object of the present invention is to provide a method for producing a mold for minute pattern transfer, which is suitable for production of an optical component such as a diffraction grating used for general products including an organic EL element and the like, a method for producing a diffraction grating using the obtained mold, and a method for producing an organic EL element using such diffraction grating. Another object of the present invention is to provide the mold for the minute pattern transfer, the diffraction grating, and the organic EL element, using the producing methods as described above.